First repeated character

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Collections;

namespace FirstRepeatedCharacter1
{
class Program
{
static void Main(string[] args)
{
Console.WriteLine("Enter the string");
string str = Console.ReadLine();

if (str == "")
{
Console.WriteLine("string is empty");
Console.ReadLine();
return;
}

bool repeated = false;

Hashtable strhash = new Hashtable();

foreach (char ch in str)
{
if (strhash.ContainsValue(ch))
{
Console.WriteLine("First non repeated character: " + ch);
repeated = true;
break;
}
else
{
strhash.Add((int)ch, ch);
}
}

if (repeated == false)
{
Console.WriteLine("No repeated characters");
}

Console.ReadLine();
}
}
}